Increasing state ownership and rising resource nationalism are emerging as the main long-term threats to global oil supplies, according to a report for the industry by an energy consultancy.

The report by PFC Energy highlights the shift in power towards state-controlled national oil companies.

Multinationals own or have access to less than 10 per cent of world oil resources.

Resource nationalism, which is limiting access for international oil companies, and failure by the national oil companies to reinvest profits in production are limiting outlay required for new global capacity to replace existing resources, which are being substantially depleted.

Robin West, chairman of PFC Energy, said: "The full impact of the nationalisations that took place in the 1960s and 1970s are taking effect now.".

The PFC study shows political factors are limiting production capacity increases in Mexico, Venezuela, Iran, Iraq, Kuwait and Russia. Saudi Arabia is also limiting capacity expansion but because of a self-imposed cap, not management challenges, like the other countries.

These seven countries account for 65 per cent of the world's reserves and 45 per cent of crude oil production. Before 1961 the industry could invest almost anywhere, except the Soviet Union and Mexico.

Then it was pushed out of the Middle East and Venezuela. Investment by international companies shifted to the North Sea, north slope of Alaska and offshore, but the North Sea and Alaska are maturing even as output in key producers is declining.

Jim Mulva, chief of Conoco-Phillips, said: "To the extent that access has become more restricted for the international oil companies, it could lead, and may lead, to constraint on supply."

However there is little the oil companies can do to change that. "For international oil companies, access is a real challenge," Mr Mulva said.

PFC notes that the Cantarell field, which accounts for two-thirds of Mexico's production, is declining rapidly, yet developing deep-water exploration could hold production steady if not boost it.

Venezuela could significantly increase production if it encouraged investment in heavy crude, yet its move this month to nationalise major fields is likely to have the reverse effect, as the international oil companies get less for their investment.